GetPixel always returns CLR_INVALID

Question

I'm trying to read the pixel color of a given window. The window is not mine. The window may not have focus or be at foreground. I don't know if it matters, does it? I have the window handle, so I do:

  HDC hdc = GetDC(m_window);
  if (hdc) 
  {
    COLORREF color = GetPixel(hdc,x,y);

    if(color == CLR_INVALID)
    {
      wxLogDebug("COLOR DATA INVALID");        
    }
    else
    {
      wxLogDebug("COLOR DATA 0x%x", color);
    }

x and y are screen coordinates that lie inside the window. Don't know why this is not working. Any ideas?

Answer 1

You should convert x, y into client coordinates with ScreenToClient Function.